In many cases, an electric power system comprises a direct voltage rail, one or more battery elements for supplying electric energy to the direct voltage rail, and one or more power converters for converting the direct voltage of the direct voltage rail into voltages suitable for one or more loads of the electric power system. The electric power system can be for example an electric power system of a ship in which case the loads of the electric power system may comprise one or more propulsion motors, an alternating voltage network of the ship, and other loads such as e.g. one or more bow thruster motors. The motors are advantageously alternating current “AC” motors and the corresponding power converters are inverters for converting the direct voltage of the direct voltage rail into alternating voltages suitable for the AC-motors.
In many cases it is advantageous that the direct voltage of the direct voltage rail is higher than the direct voltages of the battery elements. In these cases, each of the battery elements is typically connected with a voltage-increasing power converter, i.e. a boost converter, to the direct voltage rail. The power converter comprises typically an inductor coil whose first pole is connected to the respective battery element, a controllable switch between the ground and the second pole of the inductor coil, and an unidirectionally conductive component, e.g. a diode, for providing a path for electric current from the inductor coil towards the direct voltage rail in response to a situation in which the controllable switch is in a non-conductive state.
In an electric power system of the kind described above, there is typically a need to disconnect a voltage-increasing power converter from the direct voltage rail in fault situations where the direct voltage of the voltage rail drops below the battery voltage. A straightforward approach is to connect an over-current protector, e.g. a fuse, between the voltage-increasing power converter and the direct voltage rail. An inherent challenge related to this approach is that fault current which is needed for blowing the fuse, or for activating another over-current protector, flows through the inductor coil of the voltage-increasing power converter, and thus appropriate arrangements are needed for preventing and/or for protecting against harmful voltage peaks caused by abrupt changes in the above-mentioned fault current. Furthermore, there is a need to design the inductor coil and the unidirectionally conductive component, e.g. a diode, in accordance with the fault current that can be significantly higher than corresponding electric current in normal operation. Another approach is to provide the voltage-increasing power converter with circuitries arranged to ramp down the electric current of the inductor coil in response to a fault situation of the kind mentioned above. The above-mentioned circuitries, however, increase the complexity and the costs of the voltage-increasing power converter.